位于四川盆地东部开江—梁平海槽西南侧台地边缘的长兴组礁灰岩，沉积时具有很高的原生孔隙度，但由于缺乏大规模白云石化作用，现今多数礁灰岩致密无孔。研究表明礁灰岩缺乏白云石化作用可能与早期白云石化流体供应不足有关，而并非如前人推测的是由于早期海水胶结作用将孔隙完全充填堵塞造成白云石化流体无法进入所致。早期海水胶结作用结束后，长兴组礁灰岩中仍存在大量孔隙及流体活动空间，但由于礁灰岩（礁核相）位于台地边缘靠近海槽的高能区，受早期沉积环境的控制，成岩流体为活跃循环的正常海水，不具备发生白云石化作用所需的轻度咸化海水渗透回流的条件。埋藏成岩环境下块状亮晶方解石的胶结作用是造成礁灰岩中孔隙完全丧失的原因。

The Upper Permian Changxing reef limestones deeply buried in the platform margin by the southwe st side of Kaijiang-Liangping Trough in the east of Sichuan Basin. These limestones are commonly tight due to paucity of large-scale dolomitization though they possessed high depositional porosity. It is shown that the paucity of large-scale dolomitization accounts for dolomitization fluid in short supply during early diagenesis rather than what was supposed by previous studies that dolomitization fluid could not enter the pores blocked by early cements. In the light of petrography, geochemistry and fluid-inclusion analysis, it is revealed that the facies-selective dolomitization in Changxing reef limestone implies its relation with sedimentary environment. The reason why pores are hardly preserved in the reef limestones may owe to the cementation of the radiaxial calcite precipitated from normal sea water during early diagenetic environment and the blocky sparry calcite formed during deep-buried diagenetic environment though a great amount of pores and space of fluid activity were stil present within reef limestone at the end of early marine cementation. Because the Changxing reef limestones (reef cores) are just located in high energy zone in the platform margin nearby the trough, the diagenetic fluid is actively circulating normal seawater under the control of sedimentary environment, where seepage reflux of mildly-evaporating seawater was not provided for dolomitization.